1 PM PST MON FEB 18 2008 WEATHER SYNOPSIS FOR MONDAY AND TUESDAY A moderate to strong upper ridge continues its slow eastward progression over the region mid-day Monday, with the ridge axis expected to move over the Cascades Monday afternoon and into eastern Washington Monday night. This should provide another afternoon of mostly fair skies and high freezing levels on Monday. While relatively light ridgetop winds are likely, a moderate offshore surface pressure gradient across the Cascades should provide somewhat windy and cooler conditions in lower terrain near the Cascade passes where a moderate temperature inversion is likely. As the ridge moves further east Tuesday, a slowly increasing southerly flow aloft should move over the area ahead of a strongly splitting offshore trough. This should produce gradually increasing mid and high level moisture along with slowly increasing ridgetop winds on Tuesday. As the jetstream associated with this split upper trough moves southward and sends strongest associated energy with the disturbance to the south of the region late Tuesday, heaviest precipitation with the splitting and stretching front is expected to move onshore in north-central California and southern Oregon late Tuesday. However, the northern part of the associated front should still slowly rotate northeastward late Tuesday afternoon and night, with some light rain or snow reaching the Mt Hood area and extreme southern Washington Cascades Tuesday night and the Mt Rainier area by early Wednesday. This generally very light precipitation should be accompanied by continued lowering freezing levels but only a further slight increase in winds. WEATHER FORECAST FOR MONDAY AND TUESDAY * OLYMPICS- Monday afternoon and night: a few high clouds at times, otherwise mostly fair and warm Tuesday morning: continued warm with gradually increasing high clouds Tuesday afternoon: cooler with variable mid and high clouds Tuesday night: becoming mostly cloudy with occasional light rain or snow developing * WASHINGTON CASCADES NEAR AND WEST OF THE CREST- Monday afternoon and night: a few high clouds at times, otherwise mostly fair and warm Tuesday morning: gradually increasing high clouds Tuesday afternoon: a little cooler with variable mid and high clouds Tuesday evening: cooler and partly cloudy north with increasing clouds south Tuesday night: light rain or snow developing in the south and slowly spreading northward * CASCADE PASSES, INCLUDING STEVENS, SNOQUALMIE AND WHITE PASSES- Monday afternoon and night: a few high clouds at times, otherwise mostly fair and warm higher terrain, but continued breezy and cooler lower elevations Tuesday morning: continued warm higher terrain with gradually increasing high clouds; winds decreasing lower terrain and continued cool Tuesday afternoon: a little cooler with variable mid and high clouds Tuesday evening: cooler and partly cloudy north with increasing clouds south Tuesday night: light snow developing in the south and slowly spreading northward * EAST SLOPES WASHINGTON CASCADES- Monday afternoon and night: a few high clouds at times, otherwise mostly fair and warm higher terrain; fair and cooler lower elevations Tuesday morning: warm with gradually increasing high clouds Tuesday afternoon: variable mid and high clouds increasing in the south Tuesday evening: cooler and partly cloudy north and mostly cloudy south Tuesday night: occasional light rain or snow developing in the south and slowly spreading northward * MT HOOD AREA- Monday afternoon and night: a few high clouds at times, otherwise mostly fair and warm Tuesday morning: warm with gradually increasing high clouds Tuesday afternoon: a little cooler with mid and high clouds increasing and lowering late afternoon with chance light showers Tuesday night: cooler with light rain or snow FREEZING LEVELS-CASCADE MTNS 11,000 ft N, 11,500 ft S Monday afternoon 10,500 ft N, 11,000 ft S Monday night 10,000 ft N and S Tuesday morning 9500 ft N and S mid-day SNOW LEVELS-CASCADE MTNS 8000 ft N, 7000 ft S Tuesday afternoon 7000 ft N, 5000 ft S Tuesday evening 4500 ft N, 3500 ft S late Tuesday night and early Wednesday, except snow levels locally near the surface Cascade passes Monday and Tuesday FREEZING LEVELS-OLYMPIC MTNS 11,000 ft Monday afternoon 10,000 ft Monday night 9000 ft Tuesday morning 8000 ft mid-day SNOW LEVELS-OLYMPIC MTNS 6500 ft Tuesday afternoon 5500 ft Tuesday evening 4500 ft early Wednesday Cascade Snow/Freezing Levels refer to the northern Washington Cascades (N) through Mt Hood area (S). Central Washington Cascade snow levels (typically near Snoqualmie Pass) are normally midway between indicated N and S levels. Note that surface snow/freezing levels are common near the passes during easterly pass flow and may result in multiple snow/freezing levels. 24 HOUR FORECAST OF PRECIPITATION IN INCHES OF WATER EQUIVALENT ENDING AT 4AM TUE WED * HURRICANE RIDGE 0 LT .10 * MT BAKER 0 0 * WASHINGTON PASS 0 LT .10 * STEVENS PASS 0 LT .10 * SNOQUALMIE PASS 0 LT .10 * MISSION RIDGE 0 LT .10 * CRYSTAL MTN 0 LT .10 * PARADISE 0 LT .10 * WHITE PASS 0 LT .10 * MT HOOD 0 .25 WINDS IN MILES PER HOUR (MPH) * CASCADE PASS LEVEL WINDS E 10-20 with occasional higher gusts through Tuesday morning E 5-15 Tuesday afternoon and night * FREE WINDS AT 5000 FT SE 5-15 N, 15-25 S and Olympics Monday afternoon S 10-20 Cascades, 15-30 Olympics Monday night S 10-15 Cascades, 20-40 Olympics Tuesday morning S 5-15 Cascades, 15-30 Olympics Tuesday afternoon S 5-15 N and C, SW 15-25 S and S 20-30 Olympics Tuesday night * FREE WINDS AT 9000 FT S-SE 5-15 N, 10-20 S and 15-30 Olympics mid-day S 15-30 Cascades, 20-40 Olympics Monday mid-late afternoon S 15-30 N, 20-40 S and Olympics Monday night S 20-35 Tuesday morning S 15-25 Cascades, 20-30 Olympics Tuesday afternoon S 10-20 Cascades, 15-30 Olympics Tuesday night EXTENDED WEATHER SYNOPSIS FOR WEDNESDAY THROUGH FRIDAY In general, the extended forecast period should be dominated by a continuing strong split in the upper level flow over the western US. As a split upper trough moves mostly to the southeast of the area into north-central California on Wednesday, the northern part of the associated weak front should further weaken and mostly dissipate as it moves into north-central Washington and the still slowly retreating upper ridge Wednesday. This should spread some very light rain or snow northward to about Snoqualmie Pass Wednesday morning before the front weakens to produce little more than light showers later Wednesday morning and mid-day. Decreasing clouds and showers are expected Wednesday afternoon and night as weak riding rebuilds ahead of a slightly stronger disturbance expected to reach the coast early Thursday. The main Pacific jetstream should remain well to the south Thursday with a weaker southerly flow over the Northwest moving out of a broad and deep upper trough moving into the Gulf of Alaska. This should allow another weak frontal system to rotate slowly northeastward over the region Thursday morning and early afternoon. This should spread increasing light to occasionally moderate rain or snow northward Thursday morning along with increasing winds and lowering freezing levels. Following the front light to moderate showers should decrease later Thursday with some partial clearing Thursday night into early Friday. Another similarly weak front, with the main energy well south, should slowly spread generally light snowfall northward Friday afternoon through early-mid Saturday before decreasing to light showers late Saturday. However, with the strongest westerly flow remaining across California, relatively light snowfall amounts are likely at relatively low and lowering freezing levels. EXTENDED FORECAST FOR WEDNESDAY THROUGH FRIDAY Wednesday morning: occasional light rain or snow south of Snoqualmie Pass with partly cloudy skies north Wednesday afternoon and night: partly cloudy with decreasing light showers in the south Thursday morning: increasing clouds with light to occasionally moderate rain or snow spreading northward Thursday afternoon: light rain or snow decreasing and becoming more showery Thursday night and Friday morning: partly cloudy with decreasing light snow showers Friday afternoon: increasing clouds with light rain or snow developing southern Washington Cascades and Mt Hood and spreading northward Friday night: light rain or snow * SNOW LEVELS 3-5000 ft N, 2-4000 ft S early Wednesday 2-4000 ft N, 3-5000 ft S late Wednesday and early Thursday, except snow levels near the surface Cascade passes Wednesday and early Thursday, rising to near free air levels mid-day Thursday 2-4000 ft N and S mid-late Thursday 1-3000 ft N, 3-5000 ft S Friday morning 2-4000 ft N and S Friday afternoon and night